Blow molding is a well-known process for making plastic products, in particular containers or hollow bodies such as fuel tanks. The process usually encompasses extruding a parison, which may consist of a plurality of layers of different material, and transporting the still hot parison to an open blow mold, where it is disposed between the blow mold halves. The blow mold halves are then closed, capturing the parison between them, and the interior of the parison is subjected to a pressure medium, thereby inflating the material of the parison against the inside walls of the blow mold and shaping it accordingly. Especially in the case of fuel tanks there is often a desire to form not only the tank itself, but also to position on the inside of the tank components of the fuel system, for example a fuel pump, a fuel filter, baffles to prevent or reduce sloshing of the fuel during operation of the vehicle, hoses for fuel and venting, and cabling for the electrical supply to the fuel pump. This poses some practical challenges.